Astigmatism is a defect in the eye that is corrected by a lens with an assymetric prescription. The assymetric prescription, which is usually expressed as cylinder on the patients' prescription order, causes at least a portion of the surface of the lens to have the shape of a toric segment. Hence, such lenses are called toric lenses. The assymetric prescription must be correctly oriented with respect to the eye of the wearer. For ordinary eyeglasses, this presents no problem, because the lens is permanently fixed to the frame at the correct orientation, and the frame is non-rotatably attached to the wearers' face by the earpieces and nosepiece. For a toric contact lens, orientation is not so simple.
One method of maintaining correct orientation of the lens is to construct the lens with its intended bottom third thicker than its intended top two thirds. The blinking of the patients' eyelid tends to push the thicker intended bottom of the lens to the bottom of the eye. However, because of irregularities in the shape of the cornea, interference by the lower lid, etc., the intended bottom of the lens does not always settle at the exact bottom of the eye. Another way to maintain lens orientation is to construct the lens with a relatively thick central zone and thinner top and bottom zones. However, lenses of this type are also capable of settling to a position that is different from that intended. Very often a lens of either type settles to a position that is rotated 5 or more degrees from its intended position. This rotation must be measured and taken into account in the cylinder portion of the lens prescription.